Integrated electrical circuits and other microstructured components are conventionally produced by applying a plurality of structured layers onto a suitable substrate which, for example, may be a silicon wafer. In order to structure the layers, they are first covered with a photoresist which is sensitive to light of a particular wavelength range, for example light in the deep ultraviolet (DUV) spectral range. The wafer coated in this way is subsequently exposed in a projection exposure apparatus. A mask, which contains a pattern of structures, is thus illuminated by an illumination system and imaged onto the photoresist with the aid of a projection objective. Since the imaging scale is generally less than one, such projection objectives are often also referred to as reducing objectives.
After the photoresist has been developed, the wafer is subjected to an etching process so that the layer becomes structured according to the pattern on the mask. The photoresist still remaining is then removed from the other parts of the layer. This process is repeated until all the layers have been applied on the wafer.
The mirrors used in projection exposure apparatus generally include a reflection coating, which is made up of a plurality of individual layers and whose reflection coefficient is often more than 90%. Lenses and other refractive optical elements, on the other hand, are provided with antireflection coatings in order to reduce light losses and imaging errors due to undesired double reflections at the interfaces of the refractive optical elements.